Electrical braking systems are considered to be the alternative which will replace conventional braking systems in the future. Conventional braking systems (e.g., in commercial vehicles) transmit a signal indicating the desire to apply the brakes (actuation of the brake pedal by the driver) through pneumatic or hydraulic systems. Electrical braking systems (EBS), on the other hand, transmit this braking signal by means of electrical conductors. The actual braking force in either system is supplied by means of one or more hydraulic or pneumatic brake cylinders. EBS provides several advantages including increasing the speed of brake signal transmission. Additionally, an EBS can account for important braking factors such as the load, brake lining wear, an antilock braking system, etc.
The braking system is by its nature the most important element to ensure the travel safety of the vehicle. For this reason, isolated, i.e., separated, storage devices are required for storing pneumatic or hydraulic energy as well as electrical energy (batteries). This requirement applies even to fully electronic dual circuit braking systems.
Electrical energy storage devices must be recharged in accordance with this criterion from a common energy source or charging device (generator). For safety reasons, energy must flow only from the charging device to the electrical energy storage devices so that a malfunction, such as a short circuit in one electrical energy storage device, will not have an adverse effect on the other electrical energy storage device.
A voltage supply circuit of the type described above is disclosed in German patent document DE-A 35 02 100 published on Jul. 31, 1986. This reference discloses a voltage monitoring system for an auxiliary battery. In the case of a battery failure, a switching device switches the supply line of both circuits to the working battery so that the working battery supplies power to both of the circuits.
A disadvantage of this prior art circuit is that the charging of the auxiliary battery is not always assured if the starter battery fails. Furthermore, the diode circuit used for the separation of the two batteries consumes a relatively large amount of energy. A diode circuit connected between the charging device and the auxiliary battery is essentially disadvantageous because the charging voltage received by the auxiliary battery from the main power circuit is reduced by the potential drop across the diode circuit. As a result, the auxiliary battery is only partially charged.
It is therefore the object of the present invention to provide a dual voltage supply circuit for vehicles which safely separates, and uniformly charges, the first (starter battery) and second (auxiliary battery) charge storage devices.